The present invention relates to a backup control method and system for an external memory in a data processing system. More particularly, it relates to a method and system for copying data for the backup of an external memory onto another storage medium.
In a data processing system which includes a central processing unit (CPU) and an external memory, for example, a magnetic disk device, data stored in the external memory is used for data processing by the CPU, and it is renewed or modified according to the result of the data processing. Meanwhile, there is always the fear that such data in the external memory will be destroyed by failure of the hardware, an error in the program, an error in the operation, the mischief of a hacker, or any other unexpected situation. In order to recover the data thus destroyed, it is common practice nowadays to copy or dump the contents of the external memory onto another storage medium, for example, a magnetic tape device at suitable time intervals.
The simplest dumping method is to dump all the contents of the external memory, such as a magnetic disk device; into the magnetic tape device or the like periodically. With this method, however, the period of time for which ordinary data processing is suspended due to the dump processing is long, and besides, a storage medium having a large storage capacity is required for storing the dumped data. To the end of eliminating these difficulties, in a system disclosed in the official gazette of Japanese Patent Application Laid-open No. 90770/1982, a disk control device records information items indicative of the positions (for example, tracks) of renewed or modified data blocks in a control memory, a CPU reads out the contents of the control memory and dumps only the renewed data blocks into a magnetic tape device after all jobs have been executed, and the CPU resets the control memory when the dumping has ended. Using the renewed data blocks thus obtained and all data items previously dumped, data can be recovered.
The aforementioned system disclosed in the official gazette has the merit that the period of time and the capacity of the storage medium as required for the dumping are remarkably shortened and reduced. However, no job can be processed until the dumping ends. The reason is that, if data is renewed in the course of the dumping, the renewal might be missed, so the renewed data might not be dumped. By way of example, in a case where data on a certain track is renewed immediately after data on the same track has been dumped, the record of the data renewal is erased by the reset operation at the end of the dumping. Meanwhile, 24-hour operations with no cessation are often required of computer systems in computing centers and computer systems for various sorts of on-line processing. For such a computer system, it is very unfavorable to stop the ordinary job processing for a considerable time interval on account of the dump processing.